A material having properties that do not exist in nature can be constructed artificially by arranging chips (unit structures) of metal, dielectric material, magnetic material, superconducting material, and so on at sufficiently short intervals relative to the wavelength (no more than approximately one tenth of the wavelength). This type of material belongs to a larger category than a category of materials found in nature, and is therefore known as a metamaterial. The properties of a metamaterial vary according to the shape, materials, and arrangement of the unit structure.
Among such metamaterials, a metamaterial in which an equivalent permittivity ∈ and a permeability μ are simultaneously negative is known as a “left-handed material (LHM)” since the electric field, magnetic field, and wave vector thereof form a left-handed system. In this specification, a left-handed material is referred to as a left-handed metamaterial. In contrast, a normal material in which the equivalent permittivity ∈ and permeability μ are simultaneously positive is known as a “right-handed material (RHM)”. As shown in FIG. 1, a relationship area between the material and the permittivity ∈ and permeability μ can be divided into first through fourth quadrants corresponding to the sign of the permittivity ∈ and the sign of the permeability μ. A right-handed material is a material belonging to the first quadrant, and a left-handed material is a material belonging to the third quadrant.
A left-handed metamaterial possesses particularly idiosyncratic properties such as the existence of a wave (known as a backward wave) in which the signs of the group velocity (the speed at which energy is propagated) and phase velocity (the speed at which a phase advances) of the wave are reversed, and evanescent wave amplification, an evanescent wave being a wave that decays exponentially in a non-propagation area. A line that transmits backward waves generated by a left-handed metamaterial can also be constructed artificially. This is described in the following Non-Patent Document 1 and Non-Patent Document 2, and is therefore well known.
A line on which backward waves are propagated by arranging unit cells constituted by a metallic pattern periodically has been proposed on the basis of this concept of left-handed material construction. This transmission characteristic has been handled theoretically up to the present time, and hence the facts that the line possesses a left-handed transmission band, a band gap occurs between the left-handed transmission band and a right-handed transmission band, the width of the band gap can be controlled in accordance with reactance in the unit cell, and so on have become theoretically evident. These points are described in the following Non-Patent Document 3.    [Non-Patent Document 1]    D. R. Smith, W. J. Padilla, D. C. Vier, S. C. Nemat-Nasser, and S. Schultz, “Composite medium with simultaneously negative permeability and permittivity,” Phys. Rev. Lett., vol. 84, no. 18, pp. 4184-4187, May 2000    [Non-Patent Document 2]    C. Caloz, and T. Itoh, “Application of the transmission line theory of left-handed (LH) materials to the realization of a microstrip LH line”, IEEE-APS Int'l Symp. Digest, vol. 2, pp. 412-415, June 2002    [Non-Patent Document 3]    Atsushi Sanada, Christophe Caloz and Tatsuo Itoh, “Characteristics of the Composite Right/Left-Handed Transmission Lines,” IEEE Microwave and wireless Component Letters, Vol. 14, No. 2, pp. 68-70, February 2004
Left-handed metamaterials can be broadly divided into resonant materials and non-resonant materials, depending on the constitution thereof. The initially created left-handed metamaterials were resonant. A resonant left-handed metamaterial uses an area in which both the permittivity of an artificial dielectric material and the permeability of an artificial magnetic material are negative in the vicinity of the resonance frequency. Therefore, this type of material is disadvantaged in that the frequency bandwidth in which the material functions as a left-handed material is narrow. Moreover, since a frequency in the vicinity of the resonance frequency is used, an increase in loss occurs.
In contrast, a non-resonant left-handed metamaterial is based on a transmission line characteristic according to which the distributed inductance (L) and the distributed capacitance (C) of the transmission line of a normal medium are reversed. In a transmission line having reversed distributed constants LC, the aforementioned backward waves are transmitted, and therefore the line functions as a left-handed metamaterial. The frequency bandwidth in which a non-resonant left-handed metamaterial functions as a left-handed material is wider than that of the resonant left-handed metamaterial, and therefore a reduction in loss is achieved.
A transmission circuit employing a lumped constant LC element (a chip inductor, a chip capacitor, and so on) and a distributed constant type material in which periodical structures are disposed on a transmission line have been used as non-resonant left-handed metamaterials. However, there is an upper limit to the operation frequency of a material employing a lumped constant LC element (operations are only possible at or below the self-resonant frequency of the element), and it is therefore difficult to realize a left-handed metamaterial that operates at or above several GHz. Further, this type of material uses a large number of lumped constant LC elements, and is therefore difficult and expensive to manufacture. As regards distributed constant type materials, research has focused mainly on plane circuit type structures formed on dielectric substrates. However, it has not been possible up to the present time to realize a non-resonant left-handed material in relation to a radiation field rather than electromagnetic waves in the plain circuit.